The present invention relates to a process and an apparatus for the automatic production of large glass containers.
The term, "large containers", is to be understood here to mean containers having a capacity generally between 5 and 50 liters (in Italy, for example, the familiar carboys). These containers cannot be produced with the conventional machines for the production of bottles and thus are still being produced today mostly by mainly manual methods.
Prior art methods for the production of large glass containers, still used in various countries of the world, are discussed as follows.
In one manual method, a highly skilled worker extracts, by means of a steel tube, the proper amount of glass in an approximately spherical form, and then blows it by mouth while at the same time rotating the tube, until the finished object is obtained and is detached from the tube by contact with a cold iron.
This method is nowadays virtually abandoned for obvious reasons of low productivity except for "artistic" products.
A semi-automatic process, which is now to be considered the most widespread, calls for the manual extraction of the glass from a particular part of the melting furnace, and then for working by vacuum and compressed air in cast-iron molds, nearly all the movements of the cycle being performed by hand, and in any case all controlled by hand. The cycle can call for either two molds (preparatory+finishing) or three molds (preparatory+prefinishing+finishing), depending on the object to be produced.
In this method, a first worker takes a "gob" of glass from the furnace and places it in a first mold called the preparatory mold. Then a second worker undertakes to fashion the gob--that is, to make it into a parison--working with compressed air and suction. Then the parison is passed to a second mold, called the prefinishing mold, where a third worker fashions it--usually with compressed air--to bring it to an intermediate shape and size. Lastly, this parison is transferred to a third mold, called the finishing mold, where still another worker proceeds--again with compressed air--to put it into its final shape.
As it is easy to understand, this system, in addition to requiring much labor and being very slow, does not assure an end product of constant quality.
Although the workers assigned to the various operations are specialists, and their attention is particularly directed towards controlling the very gradual blowing of the glass container, for the fundamental purpose of obtaining a uniform thickness in its walls, this object is perforce achieved only partially.
The greatest difficulty is encountered in the final phase of the process, that is in the third, finishing mold--in which, for example in the case of 50-liter carboys, there is a change from a volume of about 10 liters in the intermediate parison to a final volume of 50 liters, with a very evident dilation of the parison--due to the fact that, precisely for more accurate visual control of the blowing, it is performed, at least initially, with the mold open. It is thus found that, on the side on which the mold is opened, it then happens that the glass undergoes a certain exposure to air and the cooled portion of the parison offers greater resistance to the dilation, while in the areas protected by the mold, and therefore the hotter areas, the glass wall tends to expand more rapidly and consequently to become thinner.
To prevent this problem from reaching the extreme of rupturing the glass wall in the thinnest zone, the worker must exercise, as stated, a careful control of the blowing, possibly supporting the glass wall in the areas where it might tend to sag--all a very long and difficult operation.
In addition to the manual methods described above, an automatic system, the Roirant system, is known, although it has not been widely adopted.
This process is characterized by the fact that the glass does not come from a gob feeder, but it drawn at a particular part of the furnace by the first mold. This mold is disposed with the open bottom down, perfectly flush with the surface of the molten glass, and, by means of a vacuum, sucks up the glass which thus fills it. A special knife cuts the glass flush with the mold. The mold then leaves the furnace and the glass is worked directly in a finishing mold or in an intermediate mold and in a finishing mold.
In substance, since the first mold must be considered as a substitute for the feeder, and no perceptible expansion can be performed in it, the cycles thus conceived must be considered as one-mold or two-mold cycles.
In this process, the expansion of the object is brought about by creating a vacuum between the parison and the walls of the molds.
Disregarding the manual process because of its obvious disadvantages, the semi-automatic process and the Roirant automatic process also have a number of disadvantages.
While the semi-automatic process makes it possible to produce a quality product it does not have constancy, which is typical of processes in which there is a conspicuous amount of manual intervention.
Furthermore, the ratio between productivity and labor required in the machine department is to be considered on the average of from 1/6 to 1/12 that of an automatic system.
The Roirant process also has a number of basic disadvantages which have prevented its widespread adoption, even though it is a process which is not new. First of all, the thickness of the glass in the various areas of the product is difficult to control and highly irregular. This produces many negative consequences, such as lesser strength in the bottle for an equal weight of glass, need to use a greater weight of glass. Furthermore, the productivity, whether it is evaluated on a per-mold or on a per-labor-unit basis, is low.
Another disadvantage of this process is that of being difficult to use, such that the production outputs (that is, the ratio of the net production to the gross production) are excessively low.
A further disadvantage is that of requiring a much lower height of the furnace with respect to the floor level of the plant than that required for all the other automatic hollow glassware operations (bottles and glasses). This makes this process difficult to incorporate into the average glass factory.
Another point against this process is the fact that, on account of the way in which the first mold is operated, and the knife that cuts the glass, this first mold is subject to frequent breakdown requiring frequent maintenance.
Symptomatic of the severity of these disadvantages is the fact that, where the cost of labor permits, glass making industries, prefer to use the semiautomatic process rather than the existing automatic processes.